Abstract : A microwave carrier signal emitted from a tracker unit is amplitude modulated periodically at different preset modulation frequencies during spaced intervals between established time slots. The carrier signal is detected by differently coded credential units within a reception zone whereby such signal is demodulated at the different modulating frequencies to produce reply signals at one of several different code frequencies during one of the time slots dependent on the credential unit coding. The reply signals are detected by the tracker unit during narrowed reception windows within the time slots. Such reply signals are decoded within separate frequency channels corresponding to the code frequencies thereof to extract signal data processed to indicate entry, departure and status of the credential units relative to the reception zone.